The present invention relates generally to hydrostatic transmissions (xe2x80x9cHSTxe2x80x9d) and transaxles including hydrostatic transmissions, and in particular to a bypass system for such transmissions. Specific exemplary embodiments discussed herein relate to riding lawn mowers and similar small tractors.
The description of art in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is xe2x80x9cprior artxe2x80x9d with respect to this invention, unless specifically designated as such.
Riding lawn mowers and small tractors 10 of the type discussed herein and depicted in FIGS. 1A and 1B generally use an engine having a vertical output shaft 12 that is connected to the transaxle 14 via a conventional belt and pulley system 16. A standard HST for such a transaxle includes a hydraulic pump that is driven by the engine output shaft 12, and a hydraulic motor, both of which are usually mounted on a center section. Rotation of the pump by an input shaft creates an axial motion for the pump pistons. The oil pressure created by this axial motion is channeled via porting to the hydraulic motor, where it is received by the motor pistons. The axial motion of these pistons against a thrust bearing causes the motor to rotate. The hydraulic motor in turn has an output shaft that drives the vehicle axles through differential gearing. For additional background, the reader is referred to U.S. Pat. No. 5,201,692, (xe2x80x9cthe ""692 patentxe2x80x9d) issued to Johnson et al., and incorporated herein by reference in its entirety.
A problem with the typical HST arrangement is that a xe2x80x9cneutral gearxe2x80x9d does not exist, as it is merely a point where the hydraulic pressure in the pump goes to zero. However, at this point the oil remains in the transmission, preventing the vehicle from being freely rolled. The present invention is an improvement over prior art methods used to place a transaxle into neutral gear and enable movement of the tractor without the motor running or, more generally, without the transmission being engaged.
Prior art has solved the problem by diverting the oil through a hydraulic valve from the pressure side to the vacuum side of the HST center section. The problem with such a design is that the hydraulic valve allows for the movement of only a limited amount of oil due to inherent design limitations, such as the diameter of the hydraulic valve through which the oil is diverted. Machining such a valve also requires precise tolerances, thus increasing the manufacturing costs of the unit.
The ""692 patent solves this neutral gear problem by providing a mechanism whereby the motor block is mechanically lifted from its running surface. This mechanism allows the oil to bypass the vacuum-pressure (hydraulic) circuit and to exit the case completely. This mechanism operates to enable the vehicle to free-wheel more easily than is possible with prior art hydraulic valve methods.
FIG. 1C (which is similar to FIG. 2 of the ""692 patent) is a section view through the transaxle 14. To activate the bypass feature disclosed in the ""692 patent, a bypass arm 18 is manipulated by the user to rotate a bypass actuator 20. The bypass actuator 20 includes a rod 22 which is shaped at its base 24 so that rotation of the rod 22 forces a bypass plate 26 to press against the base of the motor 29, thereby breaking its seal to the motor running surface (See FIGS. 2 and 5 of the ""692 patent). It will be apparent to those of skill in the art that a pin or multiple pins, or other mechanical means may be used to lift the motor block. These mechanical lifting mechanisms allow the oil to flow between the motor and the transmission cavity.
A bypass rod 28 is connected to the bypass arm 18 to facilitate manipulation of the bypass arm 18. FIG. 1D shows the bypass rod 28 in an unlocked position. The prior art means for activating the bypass mechanism includes pulling the bypass rod 28 through the tractor hitch plate 30 and securing it in place with a weldment (or cross pin) 32. To secure the bypass rod 28 in the hitch plate 30, the hitch plate 30 must include a relatively complex stamp out, e.g., a keyhole 34. See FIG. 1F which shows the bypass rod 28 extending through the keyhole 34 of the hitch plate 30. Perhaps more troublesome is the requirement to have tight tolerances for setting the cross pin 32 relative to the hitch plate 30 and the keyhole 34. This tolerance requirement is especially frustrating because manufacturing tolerances involving the hitch plate 30 and the vehicle frame are much looser.
While the present invention relates to hydrostatic transaxles and transmissions generally, it will be better understood within the discussion of exemplary embodiments directed toward riding lawnmowers and similar small tractors.
A primary object of the present invention is directed toward a bypass system, and in particular, a bypass latch, for a hydrostatic transaxle or transmission. A hydrostatic transaxle according to one embodiment of the present invention comprises a casing including a hydrostatic transmission located in the casing. The transmission comprises a motor on a motor running surface and a pump connected to the motor via a hydraulic circuit. A bypass actuator, in a preferred embodiment, is rotatably positioned to lift the motor from the motor running surface when the actuator is rotated such that hydraulic fluid flows out of the motor (a part of the hydraulic circuit). Alternatively, other components can be shifted to allow fluid to flow out of the hydraulic circuit. A bypass arm is preferably positioned external to the casing (though need not be) and affixed to the bypass actuator and rotatable therewith. A latch arm that is rotatable about an axis parallel to an axis about which the bypass actuator rotates is adapted to releasably engage the bypass arm. The latch arm is coupled with a brake rod such that operation of the brake rod disengages the latch arm from the bypass arm, whereby the bypass actuator is disengaged and the transmission is thereby engaged.
Accordingly, an embodiment of the invention is directed toward a bypass assembly comprising a latch arm having a latching end for releasably engaging the bypass arm and a brake end coupled with a brake rod. A returning spring, preferably an extension spring, is connected to the latch arm to apply a returning force to bias the latch arm toward an at-rest position. In a preferred embodiment the at-rest position is the position where the brake end of the latch arm abuts, or is stopped against, a brake arm. A second spring, preferably a compression spring, cooperating with the brake rod is used to apply a braking force to the brake arm when the brake rod is actuated. In a preferred embodiment, the compression spring transmits a force to the braking end of the latch arm prior to transmitting a braking force to the wheels or axles of the tractor. A slidable member adapted to control the timing of the force may be positioned on the brake rod.
One advantage of the design is that it is integral with the transmission. One method of integrating a bypass latch with a hydrostatic transmission according to the present invention comprises placing the hydrostatic transmission in a casing and connecting a bypass actuator to the transmission. This allows the transmission to roll more freely when the bypass actuator is engaged than when the bypass actuator is disengaged. A bypass arm positioned external to the casing is connected to operate the bypass actuator. A latch arm adapted to releasably engage the bypass arm is positioned external to the casing as well. The method of integrating further includes linking the latching arm to a brake rod such that operation of the brake rod causes the latching arm to release the bypass arm, whereby the bypass actuator is disengaged.
By integrating the design, the manufacturer could provide a means to actuate the bypass that is less expensive than the prior art. The current means of activation involves a bent wire form with a weldment (or cross pin). A straight wire form, rod, or stamping could be used with the proposed invention thereby reducing tolerance requirements and costs.
Another advantage of the invention is that it can be deactivated by the pressing of the brake pedal. Many vehicles require the operator to press the brake pedal prior to starting the tractor. The present invention, which deactivates the bypass actuator when the brake pedal is applied, insures that the transmission is ready for operation. This is particular useful when the user has perhaps forgotten that the bypass has been left xe2x80x9con,xe2x80x9d i.e. the transmission is disengaged.
Other objects and advantages in accordance with the present invention will be apparent to those of skill in the art from the teachings disclosed herein.